Syringes and other injection devices have different ranges of application. One of these is syringes for administration of fluids with a viscosity above 1000 mPa*s, such as a gel. For instance, a gel may be injected into the skin of a human being or an animal in order to obtain a volume augmentation of the skin. This application has been used in the esthetics industry for e.g. smoothing wrinkles and folds, shaping facial contours, lip sculpting, etc. Such gels are, for example, marketed by the applicant under the product trademarks Restylane and Perlane which are based on the so called NASHA-gel (Non-Animal Stabilized Hyaluronic Acid). Another application is for treating female urine incontinence wherein an injection of the gel is performed. The applicant markets such gel under the product trademark Zuidex (also a NASHA-gel). Another NASHA-gel product under the trademark Durolane is injected into the bone of a patient for treatment of e.g. knee osteoarthritis.
When a large quantity of fluid is to be delivered the syringe should have a large capacity, or should be refilled or changed several times during the administration. The large quantity generally makes the cross section of the syringe large and thereby the area of the plunger that transfers the applied pressure to the fluid will be large as well. The larger the diameter of the plunger is, the more pressure is needed to deliver the fluid out from the syringe. If the fluid is viscous, such as in the form of a gel, this particular problem with common syringes, containing a large quantity of fluid, is becoming even worse.
U.S. Pat. Nos. 5,496,284; 6,132,400 and WO 96/10430 describe syringes for administration of anesthetic. These syringes avoid the problem of a large plunger area, by providing a first smaller chamber having a plunger with a small area and a second chamber used as a reservoir, wherein the first chamber can be filled up with fluid from the second chamber. The small plunger area reduces the force that has to be applied during administration of the anesthetic.
While these prior art syringes facilitate the administration of an anesthetic in the form of a liquid having low viscosity, they are unsuitable for the administration of relatively viscous fluids, such as gels. A drawback of these prior art syringes is that the fluid contained within the reservoir stands in communication with air. The air is introduced when fluid is drawn into the delivery chamber during proximal movement of the plunger. If a viscous fluid is mixed with air, there is a risk that the air will be entrapped within the fluid and by that administrated into the body of a human or animal being. Another problem by mixing the fluid with air is the contamination that may defect the fluid.